Surface plasma polariton (“SPP”) metallic nanostructures flanked with periodic corrugations on the exit side, or on both sides, of a metallic thin film have been investigated for super imaging. Enhancement of transmission is derived by the plasmonic nanostructures. Beam shaping may be achieved by using the nanostructures so that it can be used as a super-lens for nano-focusing. However, to realize a true far-field super-resolution imaging and/or focusing a special coupling mechanism is needed to convert a large band of the enhanced evanescent waves, or SPP wave, to propagating waves in free space.
Surface plasmon polaritons are collective electronic excitations, or charge density waves. They are characterized by intense electromagnetic fields confined to the surface. They have many uses. For instance, an SPP wave can boost the transmission of light through sub-wavelength hole-arrays in metal films. However, its near-field signal is too weak to be collected by conventional far-field optical systems, and the limited propagation depth (within several tens of nanometers only) limits its application and further development. SPP-based systems may be used in data storage, semiconductors, near-field scanning microscopy, biomedical and sensing. However, for this to be the case the weak near-field signal problem must first be overcome.
Diffractive elements may work at near field in scanning near-field optical microscope systems to replace conventional optical fiber probes. However, controlling the constant working distance between the planar diffractive element and sample surface is difficult in practice.
For conventional zone plates working in the far-field region, resolution (Rayleigh criterion for resolution) of two point sources by a zone plate or resolving power is determined that for N<200, the limit of resolution is greater than that of a lens with the same aperture diameter D, if the zones which contribute light with a positive phase to the observation point are used, and less than that of the lens if the zones which contribute light with a negative phase to the observation point are used. Resolution on the axis is diffraction limited. It approaches the Rayleigh limit when the number of zones exceeds about 100 by which the outer diameter of the zone plate will increase significantly. The focal length of a given zone plate is determined by the incident wavelength.
An enhanced localized surface plasmon polariton (“LSPP”) based optical probe working at short wavelengths has been proposed. Such a probe may have high spatial resolution (up to 50 nm) compared with that of an optical interferometer. It is therefore possible to conduct surface measurements at much higher speeds than when using a scanning probe microscope. Scattering and field localization can be excited by metallic nanostructures within nanometer scales of distance of the sample surface. The localization and enhancement of the electromagnetic field by plasmon coupling to a metallic nanostructure may cause amplified transmission and intensity in a local region at a nanometer scale. The enhanced transmission may be derived due to the coupling of a photon to an SPP on one side of the metal, subsequent tunneling of the SPP through the nanostructures (holes or slits) to establish an SPP at the other side, and the final re-radiation into a photon. One way to couple free propagating light to surface plasmons is to have a periodic structure on the surface to satisfy conservation of energy and momentum. Therefore, a single aperture surrounded by a periodic corrugation in the metal surface will also display an enhanced transmission by the surface plasmon. As such, the near-field signal is amplified via coupling of the enhanced SPP effect in the nanostructure. The nanostructure and its related parameters depend on the application. Such a nanostructure can be termed as “nano-lens” or “super-lens”
It has been reported that the transmission and intensity of an SPP may be increased for a metallic thin film with a prescribed nanostructure. Propagation depth of the enhanced SPP can reach as long as 20 μm for the metallic thin film at a wavelength of 500 nm. Fabrication of the nanostructure may be by the use of focused ion beam direct milling. It has the advantages of high resolution, localized scanning, and one-step fabrication. Both material removal and deposition can be carried out using the focused ion beam.
Optical nano-metrology and instruments are widely used in the data storage and semiconductor industries. Commonly used apparatus and methods include optical interferometers and scanning probe microscopy. However, spatial resolution of optical interferometers does not satisfy the needs of industry as it cannot detect a feature size less than 100 nm due to the well-known limitations of diffraction. Scanning probe microscopy can increase the spatial resolution, but the measurement speed is slow and may fail to meet the requirement of mass production due to its point-by-point scanning mode. Therefore, sampling examination has been adopted to measure nano-particles of individual devices, or parts, by use of scanning probe microscopy.
Present near field imaging tools have near-field detection methods that are generally point-by-point scanning. Hence they are very slow in probing and need a relatively long time to acquire images due to the greater scanning time. Also, the probe needs to be changed often due to wear when working on irregular surfaces. Finally, the probe is easily contaminated during scanning.